The invention relates to an optical storage apparatus for recording and reproducing information by using a laser beam and to a recording and reproducing method of an optical storage medium. More particularly, the invention relates to an optical storage apparatus for recording and reproducing data at a density that is smaller than a beam diameter as is known as an MSR (Magnetically induced Super Resolution) technique and to a recording and reproducing method of an optical storage medium.
In recent years, an optical disk is being highlighted as an external storage medium of a computer. In the optical disk, by forming magnetic recording pits of a submicrometer order onto a medium by using a laser beam, a recording capacity can be remarkably increased as compared with a floppy disk or a hard disk as a conventional external storage medium. Further, information can be rewritten in a magnetooptic disk as a perpendicular magnetic storage medium using a rare earth--transition metal system material, so that its future development is more and more expected.
The optical disk of, for example, 3.5 inches has a storage capacity of 540 MB or 640 MB on one side. This means that a storage capacity of one floppy disk of 3.5 inches is equal to about 1 MB and one optical disk has storage capacity as much as that of 540 or 640 floppy disks. As mentioned above, the optical disk is a rewritable storage medium having an extremely high recording density. However, in order to get ready for the multimedia age in the future, it is necessary to increase the recording density of the optical disk even higher than that of the present optical disks. To increase the recording density, a greater number of pits have to be recorded on the medium. For this purpose, it is necessary to further decrease the pit size from the present size and to narrow the interval between the pits. In case of raising the recording density by such a method, it is further necessary to shorten a wavelength of the laser beam below the present wavelength of 670 nm. However, in case of considering the practical use, the pit size has to be reduced in the present wavelength of 670 nm. In this case, as for the recording, a pit which is smaller than the beam diameter can be formed by controlling a power of the laser beam. As for the reproduction, however, when the pit that is smaller than the beam diameter is reproduced, a crosstalk with the adjacent pit increases. In the worst case, the adjacent pit also enters the reproducing beam. When a practical use is considered, it is very difficult to use such a method.
As a method of reproducing a pit smaller than the beam diameter by the existing wavelength of 670 nm, there is a magnetooptical recording and reproducing method represented by JP-A-3-93058 and this method is known as a recording and reproducing method by the MSR (Magnetically induced Super Resolution). Presently, there are two general methods of MSR an FAD (Front Aperture Detection) system and an RAD (Rear Aperture Detection) 10 system. In the FAD system, as shown in FIGS. 1A and 1B, a recording medium is divided into a recording layer 220 and a reproducing layer 216. Data is reproduced using a reproducing magnetic field Hr applied to the medium in a state in which a laser spot 222 of a reading beam is irradiated. Depending on a temperature distribution within heating by the laser spot 222, a magnetic coupling of a switching layer 218 formed in a boundary with the recording layer 220 is released, such and such a portion of the beam spot is influenced by the reproducing magnetic field Hr and becomes a mask. On the other hand, as for a portion of the next recording pit, the magnetic coupling in the switching layer 218 is held and this 25 portion becomes an opening 224. Consequently, within the laser spot 222, only a pit 230 of the opening 224 can be read without being influenced by the adjacent pit 226. On the other hand, according to the RAD system, as shown in FIGS. 2A and 2B, an initialization to align a magnetizing direction of the reproducing layer 216 to a predetermined direction is performed by using an initializing magnet 232. The reading operation is performed by slightly raising a reproducing laser power upon reproduction. Depending on the temperature distribution within laser spot 234 of the reading beam, a mask 236 in which initial magnetization information remains and an opening 238 in which the initial magnetization information is erased and magnetization information of the recording layer 220 is transferred are formed in the reproducing layer 216. The magnetization information of the recording layer 220 transferred to the reproducing layer 216 is converted into an optical signal by a magnetooptic effect (Kerr effect or Faraday effect), so that data is reproduced. In this instance, as compared with a pit 228 of the recording layer 220 which is being read out at present, the pit 230 of the recording layer 220 to be subsequently read out is not transferred due to the formation of the mask 236 by the initial magnetization information in the reproducing layer 216. Therefore, even when the recording pit is smaller than the laser spot 234, no crosstalk occurs and the pit which is smaller than the beam diameter can be reproduced. Further, by using such a magnetically induced super resolution, since an area of the recording layer 220 except for the reproduced portion is masked by the initialized reproducing layer 216, a pit interference from the adjacent pit doesn't occur. Further, since a pit interval can be reduced and a crosstalk from the adjacent track can be also suppressed, a track pitch can be reduced and a high density can be realized even by using the existing wavelength of 780 nm.
However, in the conventional optical disk apparatus using such a magnetically induced super resolution, there is a problem that a proper reproducing operation cannot be performed unless an intensity of the reproducing magnetic field which is used upon reproduction is strictly controlled. The reason is that, for example, when the reproducing magnetic field Hr is too low in the FAD system in FIG. 1A, a forming range of the mask 226 in FIG. 1B by the magnetization of the reproducing layer 216 is reduced and the pit 228 is not masked, so that a crosstalk occurs. When the reproducing magnetic field is too strong, the forming range of the mask 226 is widened and the pit 230 is also partially masked, so that a reproducing level decreases and an error occurs. At the same time, the reproducing magnetic field Hr also acts on the recording layer 220 and there is a possibility of erasure of the recording data. When the initializing magnetic field is too low in the RAD system in FIG. 2A, an erasing range by a beam heating of the initializing magnetization of the reproducing layer 216 is widened and the forming range of the mask portion decreases, so that the pit 230 in FIG. 2B is not masked and a crosstalk occurs. When the initializing magnetic field is too strong, the erasing range by the beam heating of the initializing magnetization of the reproducing layer 216 is narrowed and the forming range of the mask 236 is widened, so that the pit 228 is partially masked, the reproducing level decreases, and an error occurs. At the same time, when the initializing magnetic field is too strong, the magnetic field also acts on the recording layer 220 and there is a possibility of erasure of the recording data. Moreover, the reproducing magnetic field and the initializing magnetic field are dependent upon the environmental temperature of the apparatus. That is, when the environmental temperature in the apparatus changes to the lower side, hysteresis characteristics of the reproducing layer become thick. In order to obtain the same magnetizing characteristics (magnetic flux density), the reproducing magnetic field has to be intensified. On the contrary, when the environmental temperature changes to the higher side, the hysteresis characteristics of the reproducing layer become thin. In order to obtain the same magnetizing characteristics, the reproducing magnetic field has to be weakened.